A liquid crystal display device has such advantageous features as a flat display element that it is thin and light-weight, and has low power consumption and low driving voltage. For this reason, the liquid crystal display device is indispensable as a display of information devices, such as lap-top personal computers, word processors, portable terminals, and portable televisions.
At present, the liquid crystal display devices that are most commonly used are the ones adopting the STN (Super-Twisted Nematic) system, and the TN (Twisted Nematic) system employing TFTs (Thin Film Transistors) as the driving elements. However, these liquid crystal display devices all adopt nematic liquid crystal, and this presents many problems to be solved to realize high-definition and large capacity displaying.
The liquid crystal display device adopting the STN system has a relatively simple structure, in which a pair of transparent substrates, each having stripe transparent electrodes, are simply faced each other, and the device is driven by a driving system a so-called simple matrix driving system. This liquid crystal display device is desirable in terms of easy manufacturing and the manufacturing costs, yet due to the limitation of such characteristics as response speed of the liquid crystal and the applied voltage versus transmittance characteristic, the development of the device has come to the level where, practically, the number of scanning lines cannot be increased any further.
Meanwhile, the liquid crystal display device of an active-matrix system adopting TFTs offers full-color dynamic images, which are compatible with that produced by cathode-ray tubes. However, in this liquid crystal display device, it is required to provide the switching elements (TFTs) without causing a pixel failure in each pixel, and manufacturing becomes harder and harder as the display capacity is increased. This liquid crystal display device also has a problem that the transmittance is lowered by the arrangement wherein the switching element is provided on each pixel.
In order to solve these problems, liquid crystal display devices adopting ferroelectric liquid crystal have been getting an attention in recent years. As disclosed in Appl. Phys. Lett. 36(1980) pp. 899-901 (N. A. Clark and S. T. Lagerwall), the ferroelectric liquid crystal has desirable characteristics such as memory effect, high response speed, and wide viewing angle. For this reason, the liquid crystal display device of a simple-matrix system adopting ferroelectric liquid crystal is more suitable for high-definition displaying with large capacity pixels.
A conventional liquid crystal display device adopting ferroelectric liquid crystal is provided with a pair of electrode substrates facing each other and a liquid crystal layer of ferroelectric liquid crystal formed therebetween. One of the electrode substrates has an arrangement wherein a plurality of transparent signal electrodes are placed parallel to one another on a surface of a glass substrate, and a transparent insulating film and a transparent alignment film are formed thereon. The other electrode substrate has an arrangement wherein a plurality of transparent scanning electrodes are provided on a surface of a glass substrate so that the scanning electrodes are orthogonal to the signal electrodes, and a transparent insulating film and a transparent alignment film are deposited thereon. The alignment films are subjected to a uniaxial aligning process such as rubbing.
The ferroelectric liquid crystal is filled in a spacing formed between the glass substrates combined with each other by a sealing material. The glass substrates are sandwiched by a pair of polarizing plates which are so positioned that their polarization axes are orthogonal to each other. Between the alignment films are provided spacers as required.
As shown in FIG. 7, a molecule 51 of the ferroelectric liquid crystal has spontaneous polarization 52 in a direction orthogonal to its long axis direction. Also, the molecule 51 is subjected to a force proportional to the vector product of (i) an electric field generated by the driving voltage applied between the signal electrodes and the scanning electrodes and (ii) the spontaneous polarization 52, and is moved along the surface of cone trajectory 53.
Thus, to the observer, as shown in FIG. 8, the molecule 51 is perceived as though it is switching between positions P.sub.a and P.sub.b on the axes of liquid crystal trajectory. For example, when the pair of polarizing plates are positioned so that their polarization axes coincide with the direction of the arrow A-A' and the direction of the arrow B-B' of FIG. 8, respectively, a dark display is obtained when the molecule 51 is in the position P.sub.a and a light display produced by birefringence is obtained when the molecule 51 is in the position P.sub.b.
The aligning states of the molecule 51 in the positions P.sub.a and P.sub.b are equivalent in terms of elastic energy, and for this reason the aligning states, that is, the optical states, are maintained even after the electric field is removed. This is called the memory effect, and is the unique property of the ferroelectric liquid crystal, which is not found in the nematic liquid crystal.
Therefore, in the liquid crystal display device in which the ferroelectric liquid crystal having such memory effect and having a high response speed as given by the spontaneous polarization is adopted to the simple matrix system, it is possible to carry out displaying with higher definition and larger capacity.
Incidentally, as disclosed in Tomita et al., Euro Display 96:, 251, in the active-drive liquid crystal display device adopting TFTs as the driving elements, conventionally, the delay time of the driving signal is defined by an RC equivalent circuit. In the TFT liquid crystal display, it is required that a pixel frequency F.sub.p and a time constant RC per line satisfy the relationship F.sub.p.multidot.RC.ltoreq.1. F.sub.p is in the order of several ten MHz, and the corresponding RC is in the order of nano seconds. This allows the TFT liquid crystal display to display dynamic images.
Meanwhile, the passive-drive liquid crystal display has not been examined thoroughly to realize dynamic image displaying, for the reason that the time constant RC is larger than that of the TFT liquid crystal display, and that there are only a few liquid crystal materials available which exhibit high speed response. Thus, even in the ferroelectric liquid crystal display having a potential to realize dynamic image displaying, no conditions for displaying dynamic images, namely, no relationship between the delay time (time constant RC) and the width .tau. Of the driving pulse had been obtained. RC and .tau. are both in the order of micro seconds. Therefore, the dynamic image display characteristics of the TFT liquid crystal display cannot be applied to the passive-drive liquid crystal display.
Japanese Unexamined Patent publication No. 77946/1995 (Tokukaihei 7-77946) discloses a method for changing the time constant in a passive-drive liquid crystal display device. In this method, a desired time constant is given per pixel by changing the resistance value of an electrode leading to the pixel in a static-drive liquid crystal display device. Because the effective value of the driving signal is changed in accordance with the time constant, the display color can be changed without using color filters.
When driving the liquid crystal display device by application of a voltage, the capacity formed between the electrodes (signal electrodes and scanning electrodes) and the substrates are charged and discharged repeatedly. Here, a charge is stored in and released from the capacity, and as a result a current flows into the liquid crystal cell via the electrodes. Also, the voltage applied to the liquid crystal is lower than the voltage applied to the input terminal of the electrodes due to the voltage drop caused by the resistance of the electrodes. Generally, the amount of voltage drop differs in different portions of the electrodes, and the voltage drop is largest at the ends of the electrodes.
For this reason, the amount of charge stored in the liquid crystal cell, that is, the current becomes relatively small. By this cause-and-effect relationship, the current value flowing through the light crystal cell and the voltage actually applied to the liquid crystal cell are different depending on the electric capacity and the electrode resistance of the liquid crystal cell.
In general, when the time constant RC as represented by the product of the electric capacity C and the electrode resistance R of the liquid crystal cell is large, and when the driving frequency is high, charge and discharge of the liquid crystal cell become insufficient, and as a result a large voltage drop results. Thus, when a rectangular driving pulse is applied to the liquid crystal, the rising portion and the falling portion of the waveform of the driving pulse are damped by the voltage drop.
Meanwhile, a liquid crystal cell adopting surface-stabilized ferroelectric liquid crystal (SSFLC) has an extremely thin cell gap of substantially 1 .mu.m. Thus, the capacity of SSFLC is larger than that of TFT cell and STN cell, and accordingly, the time constant of the SSFLC cell is larger than that of TFT cell and the STN cell. Therefore, in the SSFLC cell, the voltage drop is more prominent compared with liquid crystal cells of other types.
When driving a liquid crystal cell having a large screen at a high speed to display dynamic images, the time constant takes substantially the same value as the driving pulse width, and charge and discharge of the liquid crystal cell become insufficient. As a result, the voltage drop in the liquid crystal cell is increased to substantially 5 percent to 70 percent. When the voltage drop in the electrodes becomes larger, lowering in contrast of the liquid crystal cell, nonuniformity, and gradation failure become more prominent, which all lower the display quality.
Also, while the method of Japanese Unexamined Patent publication No. 77946/1995 (Tokukaihei 7-77946) realizes different display colors by adjusting the time constant and thus changing the voltage drop per pixel in the static-drive liquid crystal display device, this method cannot suppress voltage drop in each electrode in the liquid crystal display device of a simple-matrix type.